Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. If sufficiently severe, cardiac ischemia results in an acute myocardial infarction (AMI), also referred to as a heart attack. With AMI, a substantial portion of heart muscle ceases to function because it no longer receives oxygen, usually due to significant blockage of the coronary artery. Generally, AMI occurs when plaque (such as fat, cholesterol, and calcium) builds up and then ruptures in the coronary artery, allowing a blood clot or thrombus to form. Eventually, the blood clot completely blocks the coronary artery and so heart tissue beyond the blockage no longer receives oxygen and the tissue dies. In many cases, an AMI proves fatal because too much tissue is damaged to allow continued functioning of the heart muscle. Indeed, AMI is a leading cause of death here in the United States and worldwide. In other cases, although the AMI itself is not fatal, it strikes while the victim is engaged in potentially dangerous activities, such as driving vehicles or flying airplanes, and the severe pain and possible loss of consciousness associated with AMI results in fatal accidents. Even if the victim survives the AMI, quality of life may thereafter be severely restricted.
Often AMI is preceded by episodes of cardiac ischemia that are not sufficiently serious to cause actual permanent injury to the heart tissue. Nevertheless, these episodes are often precursors to AMI. Episodes of cardiac ischemia may also trigger certain types of arrhythmias that may prove fatal, particularly ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically, resulting in little or no net flow of blood from the heart to the brain and other organs. Indeed, serious episodes of cardiac ischemia (referred to herein as acute myocardial ischemia) typically result in either a subsequent AMI or VF, often within one to twenty-four four hours, sometimes within only a half an hour or less. Accordingly, it would be highly desirable to provide a technique for reliably detecting acute myocardial ischemia so that the victim may be warned and medical attention sought. If properly warned, surgical procedures may be implemented to locate and remove the growing arterial blockage or anti-thrombolytic medications may be administered. At the very least, advanced warning would allow the victim to cease activities that might result in a fatal accident. Moreover, in many cases, AMI or VF is triggered by strenuous physical activities and so advanced warning would allow the victim to cease such activities, possibly preventing AMI or VF from occurring.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Accordingly, techniques have been developed for detecting cardiac ischemia using implanted medical devices. In particular, techniques have been developed for analyzing intracardiac electrogram (IEGM) signals in an effort to detect cardiac ischemia. See, as examples, the following U.S. Patents: U.S. Pat. No. 5,113,869 to Nappholz; U.S. Pat. No. 5,135,004 to Adams et al.; U.S. Pat. No. 5,199,428 to Obel et al.; U.S. Pat. No. 5,203,326 to Collins; U.S. Pat. No. 5,313,953 to Yomtov et al; U.S. Pat. No. 6,501,983 to Natarajan, et al.; U.S. Pat. Nos. 6,016,443, 6,233,486, 6,256,538, and 6,264,606 to Ekwall; U.S. Pat. No. 6,021,350 to Mathson; U.S. Pat. Nos. 6,112,116 and 6,272,379 to Fischell et al; U.S. Pat. Nos. 6,128,526, 6,115,628 and 6,381,493 to Stadler et al; and U.S. Pat. No. 6,108,577 to Benser. Most IEGM-based ischemia detection techniques seek to detect ischemia by identifying changes in the elevation of the ST segment of the IEGM that occur during cardiac ischemia. The ST segment represents the portion of the cardiac signal between ventricular depolarization (also referred to as an R-wave or QRS complex) and ventricular repolarization (also referred to as a T-wave). The QRS complex usually follows an atrial depolarization (also referred to as a P-wave.) Strictly speaking, P-waves, R-waves and T-waves are features of a surface electrocardiogram (EKG). For convenience and generality, herein the terms R-wave, T-wave and P-wave are used to refer to the corresponding internal signal component as well.
A significant concern with any cardiac ischemia detection technique that relies on changes in the ST segments is that systemic influences within the patient can alter the ST segment. For example, hypoglycemia (low blood sugar levels) and hyperglycemia (high blood sugar levels) can both affect ST segment deviation. In addition, electrolyte imbalance, such as hypokalemia (low potassium levels) or hyperkalemia (high potassium levels) can affect the ST segment. Certain anti-arrhythmic drugs can also affect the ST-segment.
Accordingly, alternative techniques for detecting cardiac ischemia have been developed, which do not rely on ST segment elevation. One such technique is set forth in U.S. patent application Ser. No. 10/603,429, entitled “System And Method For Detecting Cardiac Ischemia Using An Implantable Medical Device,” of Wang et al., filed Jun. 24, 2003, which is incorporated by reference herein. Rather than examine the ST segment, the technique of Wang et al. instead examines post-T-wave segments, i.e. that portion of the cardiac signal immediately following the T-wave. In one example, the onset of cardiac ischemia is identified by detecting a sharp falling edge within post-T-wave signals. A warning is then provided to the patient. The warning preferably includes both a perceptible electrical notification signal applied directly to subcutaneous tissue and a separate warning signal delivered via short-range telemetry to a handheld warning device external to the patient. After the patient feels the internal warning signal, he or she holds the handheld device near the chest to receive the short-range telemetry signal, which provides a textual warning. The handheld warning device thereby provides confirmation of the warning to the patient, who may be otherwise uncertain as to the reason for the internally generated warning signal. Another technique for detecting cardiac ischemia based on T-waves is set forth in U.S. patent application Ser. No. 10/603,398, entitled “System And Method For Detecting Cardiac Ischemia Based On T-Waves Using An Implantable Medical Device,” of Min et al., filed Jun. 24, 2003, which is also incorporated by reference herein. With the technique of Min et al., cardiac ischemia is detected based either on the total energy of the T-wave or on the maximum slope of the T-wave. Again, if ischemia is detected, a warning signal is provided to the patient.
Hence, various cardiac ischemia detection techniques have been developed that exploit T-waves. Although these techniques are effective, it would be desirable to provide still other T-wave-based ischemia detection techniques and it is to that end that aspects of the present invention are directed. It would also be desirable to provide techniques that exploit deviations in the ST segment as well as changes in T-waves to provide further improvements in cardiac ischemia detection.
In particular, it would be highly desirable to identify particular changes in T-waves that can be used to distinguish deviations in the ST segment caused by cardiac ischemia from changes caused by hypoglycemia or hyperglycemia or other systemic affects so as to improve the reliability and specificity of ST segment-based ischemia detection. It is to this end that other aspects of the invention are directed.
Although the detection of cardiac ischemia is of paramount importance since an ischemia may be a precursor to a potentially fatal AMI or VF, it is also desirable to detect hypoglycemia or hyperglycemia so as to provide suitable warning signals and still other aspects of the invention are directed to that end. Diabetic patients, particular, need to frequently monitor blood glucose levels to ensure that the levels remain within acceptable bounds and, for insulin dependent diabetics, to determine the amount of insulin that must be administered. Conventional techniques for monitoring blood glucose levels, however, leave much to be desired. One conventional technique, for example, requires that the patient draw blood, typically by pricking the finger. The drawn blood is then analyzed by a portable device to determine the blood glucose level. The technique can be painful and therefore can significantly discourage the patient from periodically checking blood glucose levels. Moreover, since an external device is required to analyze the blood, there is the risk that the patient will neglect to keep the device handy, preventing periodic blood glucose level monitoring. For insulin-dependent diabetics, failure to properly monitor blood glucose levels can result in improper dosages of insulin causing, in extreme cases, severe adverse health consequences such as a ketoacidotic diabetic coma, which can be fatal. Accordingly, there is a significant need to provide a reliable hypo/hyperglycemia detection technique, which does not rely on the patient to monitoring his or her own glucose levels and which does not require an external analysis device.
In view of the many disadvantages of conventional external blood glucose monitoring techniques, implantable blood glucose monitors have been developed, which included sensors for mounting directly within the blood stream. However, such monitors have not achieved much success as the glucose sensors tend to clog over very quickly. Thus, an implantable device that could continually and reliably measure blood glucose levels without requiring glucose sensors would be very desirable. Moreover, as with any implantable device, there are attended risks associated with implanting the blood glucose monitor, such as adverse reactions to anesthetics employed during the implantation procedure or the onset of subsequent infections. Hence, it would be desirable to provide for automatic hypo/hyperglycemia detection using medical devices that would otherwise need to be implanted anyway, to thereby minimize the risks associated with the implantation of additional devices. In particular, for patients already requiring implantation of a cardiac stimulation device, such as a pacemaker or ICD, it would be desirable to exploit features of electrical cardiac signals, particularly ST segments and T-waves, for use in detecting hypo/hyperglycemia and still other aspects of the invention are directed to that end.